This invention relates generally to filters and, more particularly, to adaptive equalizers.
Tap drifting in an adaptive equalizer has been a problem from the very beginning of their use. A number of solutions have been proposed by those in the art for solving the tap drifting problem. For example, the use of the tap-leakage method (e.g., see R. D. Gitlin, H. C. Meadors, and S. B. Weinstein xe2x80x9cThe Tap-Leakage Algorithm: An Algorithm for the Stable Operation of a Digitally Implemented, Fractionally Spaced, Adaptive Equalizer,xe2x80x9d B. S. T. J. Vol. 61 No. 8 (October 1982) pp. 1817-1839); adding out-of-band noise (e.g., see U.S. Pat. No. 4,384,355 issued May 17, 1983, entitled xe2x80x9cControl of coefficient drift for fractionally spaced equalizersxe2x80x9d, and U.S. Pat. No. 4,376,308 issued Mar. 8, 1983, entitled xe2x80x9cControl of Coefficient Drift For Fractionally Spaced Equalizersxe2x80x9d); and processing the received signal with a whitening filter before equalization (e.g., see U.S. Pat. No. 5,751,768 issued May 12, 1998, entitled xe2x80x9cFractionally spaced equalizing circuits and methodxe2x80x9d).
Unfortunately, I have observed that tap drifting seems to be compounded in filter structures that incorporate a feedback filter such as an adaptive decision feedback equalizer (DFE). As such, I have realized that the above-mentioned solutions to tap drifting in an equalizer may not always yield the best results in the case of a DFE. Therefore, I have discovered a new method for stabilizing an adaptive filter such as a DFE. In particular, and in accordance with the principles of the invention, taps of a DFE are stabilized by adding a small amount of noise to the feedback data symbols.
In an embodiment of the invention, a decision feedback equalizer structure comprises a feedforward filter, slicer, feedback filter and further includes a noise source. The noise source adds noise to the sliced data signal (provided by the slicer) before further processing by the feedback filter. This is sufficient to stop the tap drifting and stabilize the DFE. The amount of noise added is small enough that it has a negligible effect on overall system performance.